Carbon dioxide and our oceans

The impact of man’s fossil fuel burning and deforestation on Earth’s climate can hardly have escaped anyone’s attention. But there is a second, much less known, consequence of our carbon dioxide (CO2) emissions. A large part of human-caused CO2 is absorbed by the world’s oceans, where it affects ocean chemistry and biology. This process, known as ocean acidification, is also referred to as “the other CO2 problem”.

The oceans as a sink of CO2

The oceans, covering 70% of Earth’s surface, provide a number of services to human society such as oxygen production (50% of the oxygen available in the atmosphere is produced by the oceans), source of food, income and recreation, and play a major role in the regulation of Earth’s climate. In fact, one fourth of human-caused carbon dioxide (CO2) emissions are absorbed by the oceans, translating into 24 million tons of this greenhouse gas taken up by the oceans each day. Around one third of our emissions are absorbed by the terrestrial vegetation while roughly 45% remain in the atmosphere, where their accumulation leads to climate change. It is not hard to imagine the consequences if the oceans were too lose their ability to take up part of the anthropogenic CO2 released. But what is the result of adding increasing amounts of CO2 to the ocean? A perturbation of the very chemistry of seawater — a phenomenon known as ocean acidification.

Carbon dioxide — an acid gas

The dissolution of CO2 in the ocean provokes an increase in hydrogen ions (H+), measured on the pH scale, and thus in its acidity. However, it is important to keep in mind that the oceans will never become acidic (their pH will never decrease beyond 7). The term “ocean acidification” reflects the fact that seawater pH is decreasing, and thus its acidity is increasing. The average pH of global surface waters is currently 8.1, which is 0.1 unit lower than at the onset of the industrial revolution 250 years ago. Such a small change might seem negligible, but the pH scale is logarithmic, much as the Richter scale used to measure the magnitude of earthquakes. The logarithmic nature of the scale makes this 0.1 unit change equivalent to a 30% increase in acidity. If the current CO2 emissions continue unabated, the acidity will have increased by 150% by 2100.

Impacts on marine organisms and ecosystems

The oceans are home to a myriad of species and are one of the largest sources of biodiversity on Earth. Although the research on ocean acidification is still in its infancy, results now begin to reveal a more complete, and complex, picture of the potential impacts on the marine flora and fauna. One of the most likely consequences, and the first to be discovered some 15 years ago, is the problem that organisms producing calcified structures might experience in a high-CO2 ocean. As pH drops, sea water contains less carbonate ions, a critical building block for organisms producing shells or skeletons made of calcium carbonate. This might lead to difficulties in calcification (production of these calcium carbonate structures) and perhaps even to dissolution of existing calcareous parts. Laboratory studies have shown decreases in the rate of calcification that could reach 30 to 50% in some marine organisms.

The waters around the small Italian island of Ischia provide an opportunity to picture how low ocean acidity could impact an entire marine ecosystem. At this site, the volcanic activity of Vesuvius provoke sea floor venting of nearly pure CO2 gas, creating a natural laboratory. At certain places, the acidity of these waters can be significantly lower than the global mean level, and results indicate a loss of biodiversity of 40% in areas where the acidity corresponds to levels expected by the end of this century.

Solutions

There is no doubt that ocean acidification is underway. The chemistry involved is straightforward and the changes already measurable. So what are the solutions to the problem? Most “geo-engineering” techniques that have been proposed to combat global warming will not alleviate ocean acidification, since the source of the problem remains unsolved. Such methods, e.g. the injection of sulphur particles into the atmosphere with the aim to reflect incoming solar energy, would actually probably worsen the situation as geo-engineering options would allow CO2 emissions to continue and cause further ocean acidification. The simplest, most economical, and most efficient solution to circumvent the problem remains to reduce CO2 emissions.